Pressure is mounting on the Berejiklian government to ditch its controversial plan to protect wild horses in the Kosciuszko National Park, with the International Union for Conservation of Nature warning the move would damage the state's and Australia's global reputation.

In a letter sent to Gabrielle Upton, the state's Environment Minister, the IUCN's director-general Inger Andersen said the bill to protect the animals raised "substantial issues for protected area policy and will create poor precedents for Australia and beyond".

The letter noted the national park was home to many alpine and subalpine plants and animals that "exist nowhere else on earth" that would be placed at risk by the hard-hooved, introduced horses.

The body noted the region is designated as an IUCN Category II National Park "which requires that areas will be managed in a natural state to protect large-scale ecological processes".

The "heritage horses" bill – which prioritises the historical value of the feral animals over native species, and could be approved by Parliament as soon as Wednesday – "creates a disturbing precedent at both national and global levels", the letter said.